This invention relates to an image forming apparatus. More specifically, this invention relates to an image forming apparatus for forming toner images onto a surface of a photoreceptor and transfixing the toner image onto a recording sheet.
Recently, image forming apparatus using digital electrophotographic imaging processes have been variously used for printers and copiers. The processes comprise forming of electrostatic latent image onto a surface of a photoreceptor by an irradiating light beam carrying image information corresponding to characters and images. In such digital image forming apparatus, a method for reproducing halftone images as well as solid images for forming an electrostatic latent image having a halftone-dot structure or a line-screen structure by rapidly switching a light beam between an on state and an off state has also been known and utilized for digital electrophotographic type copiers or printers. Algorithm for the method is easily realized at good cost performance.
Such image forming apparatuses comprise the steps of forming an electrostatic latent image on a photoreceptor, developing the electrostatic latent image by a dry toner for forming a toner image transferring the toner image onto a recording sheet and fusing the toner image. There are some drawbacks such as an unevenness of the density of the toner image or a low reproducibility of the halftone dots of the toner image or a low resolution of the toner image due to toner scattering during the toner transferring step.
Those drawbacks are mainly caused by the process of transferring the toner image using electrostatic phenomena of the toner particles. In other words, due to an irregular surface of the recording sheet, the surface of the photoreceptor and the surface of the recording sheet do not completely fit with each other and produce uneven gaps therebetween sufficient to cause a non-uniform electric transfer field or to induce coulomb inter-toner particles repulsive forces. Thus, image quality of the halftone images are degraded.
To avoid such drawbacks, Japanese Examined Patent Publication (JP-B) Sho. 46-41697 discloses a method for transferring a toner image visco-elastically onto a surface of an intermediate transfer medium, then re-transferring the toner image thermally from the surface of the intermediate transfer medium to a surface of an image recording sheet by melting the toner images. In this method, such degradation of the toner image does not tend to occur because the process of transferring the toner image is done through a non-electrostatic way.
Japanese Unexamined Patent Publication (JP-A) 51-94939 discloses a technique for obtaining a color image by transferring plural toner images, each having a unique hue, formed on a surface of a photoreceptor to a surface of an intermediate transfer medium such that they overlap each other, melting those toner images on the surface of the intermediate transfer medium and transferring the melted toner images onto a recording medium. In this method, such degradation of the toner image does not tend to occur because the transferring of the toner images is also done through a non-electrostatic way. Such methods for carrying out the transferring process and the fusing process simultaneously is referred to as a `Transfix` technique.